Seas are now rising faster than they have in 2,800 years, scientists say

23-02-2016

A group of scientists says it has now reconstructed
the history of the planet’s sea levels arcing back over some 3,000 years —
leading it to conclude that the rate of increase experienced in the 20th
century was “extremely likely” to have been faster than during nearly the
entire period.

“We can say
with 95 percent probability that the 20th-century rise was faster than any of
the previous 27 centuries,” said Bob Kopp, a climate scientist at Rutgers
University who led the research with nine colleagues from several U.S. and
global universities. Kopp said it’s not that seas rose faster before that –
they probably didn’t – but merely that the ability to say as much with the same
level of confidence declines.

The study was published Monday in the Proceedings of
the National Academy of Sciences.

Seas rose about 14 centimeters (5.5 inches) from 1900
to 2000, the new study suggests, for a rate of 1.4 millimeters per year. The
current rate, according to NASA, is 3.4 millimeters per year, suggesting that
sea level rise is still accelerating.

Unsurprisingly, the study blames the anomalous
20th-century rise on global warming — and not just that. It also calculates
that, had humans not been warming the planet, there’s very little chance that
seas would have risen so much during the century, finding that instead of a 14
centimeter rise, we would have seen somewhere between a 3 centimeter fall and a
7 centimeter rise.

The new work is particularly significant because, in
effect, the sea level analysis produces a so-called “hockey stick” graph —
showing a long and relatively flat sea level “handle” for thousands of years, followed
by a “blade” that turns sharply upwards in very recent times.

The discovery of such patterns itself has a long
history, going back to a 1998 study by climate researcher Michael Mann of Penn
State University and two colleagues — who found a “hockey stick” graph for the
planet’s temperature, rather than for its sea level. Since then the “hockey
stick,” in its various incarnations, has come in for voluminous criticism from
skeptics and doubters of human-caused climate change — even as multiple scientists
have continued to affirm the conclusion that the last 100 years or so are way
out of whack with what the planet has seen in the past thousand or more.

The new research also forecasts that no matter how
much carbon dioxide we emit, 21st-century sea level rise will still greatly
outstrip what was seen in the 1900s. Nonetheless, choices made today could have
a big impact. For a low emissions scenario, it finds that seas might only rise
between 24 and 61 centimeters. In contrast, for a high emissions scenario — one
that the recent Paris climate accord pledged the world to avert — they could
rise as much as 52 to 131 centimeters, or, at the very high end, 4.29 feet.

However, Kopp notes that the methods used to project
these totals may not fully capture what happens over the course of this
century. “We have a model that’s calibrated against a period when a certain set
of processes, largely thermal expansion and glaciers, were dominant,” he says,
“and we’re looking forward to a period when other factors will be dominant.”

As Kopp’s words acknowledge, the major contributors to
sea level rise in the 20th century were the melting of mountain glaciers around
the globe and the natural expansion of ocean water as it warms. However, in the
21st century, researchers think that the truly major players in potential sea
level rise, the huge ice sheets of Greenland and Antarctica, will come to play
a larger role. (Just how large remains unclear.)

The current study was based on combining a variety of
so-called “reconstructions” of past rates of sea level from 24 locations around
the world with more recent measurements from 66 global tide gauges. That’s
necessary because you can’t just measure sea level change in a single place and
get a global picture — over long time periods, factors ranging from whether
land is rising or sinking to changes in ocean currents and the gravitational
pull of the planet’s ice sheets mean that different regions can see different
amounts of sea level rise (or fall).

The new study follows in the footsteps of a 2011 study
that looked at the ocean and climate records contained in salt marshes in North
Carolina to infer the history of sea level rise over the past 2,100 years —
research that had many of the same authors. That study, too, found that the
recent sea level rise is unprecedented over that time period. (Courtesy: The
Washington Post)